Tube Amp Basics 2 -- How a Tube Amplifies

In my last post, I gave a brief overview of a standard 9-pin preamp tube (12AX7) which was intended as a basic familiarization. In this post, I’ll delve further into detail about what is happening inside the tube when it is configured as a typical gain stage found in instrument amplifiers. I’ll be touching on amplification AND distortion, which are the two primary uses of the triode in guitar amps.

This post will get a bit more technical than the last, but does not involve complex math or physics. Like the last post, it’s intended to be understood by anyone new to the technical side of amplifiers. If you are not familiar with the basic differences between AC and DC voltage, you may want to get familiar with that first for a better understanding of this material.

The Triode Amplifier

Have you ever wondered how exactly a tube amplifies your guitar signal? It’s really not that complex in all reality. As you already know if you read the last article, our 12AX7 is comprised of two triodes. Each triode can act as its own independent amplification stage. So we’re only dealing with three elements in a standard preamp gain stage.

As a review, a triode consists of a plate (or anode), a grid, and a cathode. Their physical layout is in that exact order as well. The grid sits between the cathode and the plate. Remember our schematic symbol representation of the triode?

In a triode gain stage, a positive DC voltage from our power supply is applied to the plate and the cathode is connected to ground. Mind you, there are other components at play in a real circuit, but let’s keep this simple for a moment.

So how does this triode work? Well, remember how I mentioned in the last post that the heater element heats up the cathode to the point where electrons boil off of its coating? Those negatively charged electrons begin to flow towards the positively charged plate, creating a steady flow of current. However, before they can make it to the plate, they must flow through the grid. The grid is between the cathode and plate, remember?

But let’s forget about the grid for a moment. Let’s just picture this phenomenon happening inside of our tube. Electrons are boiling off of the cathode and flowing to the plate. Pretty simple, huh? Almost a little boring.

But if we bring our grid back into the picture, and do some interesting things with it, we can actually begin to control this flow of electricity. More specifically, by applying a negative charge to the grid with respect to the cathode, we can slow down the flow of electrons.

Now if you’re new to the concepts of electricity, here’s an important point: negatively charged electrons will be attracted to positive charges. This is why the negatively charged electrons boiling off of our cathode flow to the plate, which has a positive charge applied to it. Conversely, negatively charged electrons will be repelled from negative charges. This is why applying a negative charge to our grid can slow down our flow of electrons.

The more negative the charge on our grid, the more electrons repel off of it. In other words, the more negative our grid charge is, the less electrons actually make it through to the plate. Eventually, the electron flow from cathode to plate can be completely cut off once the grid charge becomes negative enough. The grid becomes something of a brick wall and does not allow electrons through.

As our grid charge becomes more positive—or in more appropriate terms, less negative—more electrons are able to pass through to the plate.

So the grid serves as a valve of sorts (hmmm, I’ve heard that term used before), which regulates the flow of electric current from cathode to plate. If you’d like a simple visualization, think of a water faucet. When you turn the faucet valve completely off, no water flows from the water pipes in your house to the sink basin. Likewise, when you apply a negative enough charge to your triode’s grid, no electrons flow from the cathode (water pipes) to the plate (sink). When you begin to open the faucet valve, water from the pipes is allowed to start flowing past the valve to the sink. The more you open the valve, the more water flows. Likewise, when you make the charge on the grid less negative (or more positive), electrons begin to flow past it from the cathode to the plate. Making the grid charge less negative is like opening the water faucet valve more.

So where does your guitar signal fit in to all of this? In a standard triode gain stage, the signal voltage from your guitar is sent to the grid of the triode. In essence, the guitar’s voltage serves as the regulator of current flow in the triode.

If you didn’t already know, your guitar creates an AC sine wave. A picture of a basic sine wave is shown below:

As you can see, this AC (alternating current) voltage swings in both the positive and negative directions. So when it is applied to the grid of our triode, we have an ever-changing voltage swing that’s letting varying amounts of electrons flow from cathode to plate at any given moment. As your guitar signal swings more negative, less current flows through the tube. As your guitar signal swings more positive, more current flows through the tube.

The result? An amplified version of your guitar signal appears at the plate of the triode. Pretty cool, huh?

But notice how I said it was an amplified VERSION of the guitar signal. The amplified signal is not identical to the signal that was input to the grid—not exactly identical at least. Due to the operating characteristics of a tube not being perfectly linear, the amplified version of the signal will be at least slightly distorted from the original. In most cases, it’s MORE THAN slightly distorted.

Now you may or may not be confused by that. Some tube amps are obviously known for perfect clean tones, so what is meant by distortion?

There is a difference between distortion and clipping. Clipping is what is happening when we think of a typical “distortion” tone. Distortion in this context does not necessarily mean clipping. In reality, distortion as a concept simply means that the amplified waveform in this case is slightly different than the waveform that was input to the grid. It may or may not be clipped.

Let’s pretend our guitar’s signal is a perfect sine wave represented by the picture below. (The guitar signal is not perfect in reality, but we’ll pretend that it is for this example). As you can see, the signal swings from 1 volt positive to 1 volt negative. We would call this a 2 volt, peak-to-peak AC signal as it actually covers the span of 2 volts. This is actually immense for a guitar signal as a typical, hard-hit power chord with a passive pickup guitar would be more on the order of 0.1 to 0.2 volts peak-to-peak. At any rate, this is purely an example and the picture below was conveniently available with a quick Google search so that’s what we’ll use!

Let’s pretend now that this perfect sine wave is input to the grid of our triode amplifier. Let’s say just for example purposes, that the triode is configured to amplify this signal 5 times (this is an extremely low amplification factor, but again, this is just an example). Let’s also say that it does this without “clipping” the waveform. In other words, it is a “clean” amplification.

That would give us a 10 volt peak-to-peak “amplified version” of our source signal. So you might figure that our amplified version would simply be 5 volts on the positive side and 5 volts on the negative side. That would be the result if our amplifier had perfectly linear operational characteristics. But tubes don’t, and even though we would have a 10 volt peak-to-peak amplified version of our source signal, it would not be a perfectly symmetrical 5v+ to 5v- waveform.

Let’s say for our purposes that the operational characteristics of our tube create an amplified version of our input signal that is 4.2 volts on the negative swing and 5.8 volts on the positive swing. We still have a 10 volt peak-to-peak signal, but it’s now asymmetrical. The negative side of the waveform is smaller than the positive side. Our waveform is now “distorted” even though it is not clipped.

This nonlinear characteristic and the resultant distortion of the waveform are what make tube amplification so pleasing to our ears. The example given above would actually result in the addition of 2nd order harmonic overtones in the amplified version of our signal. This would make it sound more rich and 3 dimensional than the source signal.

So in other words, it’s GOOD that our signal is distorting! Even if we are playing a clean tone!

So I hope after reading this basic overview that you have more of an understanding of how a preamp tube gain stage amplifies your guitar signal.

Suckerpunch15 almost complete, and a new amp desig

The final element of my original amplifier design, the Suckerpunch15, is nearing completion. The amplifier itself is finished, but the cab still has just a little bit of work left to go.

The Suckerpunch15 will be a 1x12 combo amplifier. The cab is handmade from 3/4" Baltic Birch ply and covered with tolex.

Below is a pic during construction of the cab. As you can see, all edges are connected with half-blind dovetail finger joints.

And below is a pic of the dovetailed pieces being clamped together as the glue dries.

And finally a picture taken as the tolex covering was starting. The side you are looking at is actually the top of the cab. You can see the cutout for the amp faceplate (upward facing controls) as well as a transformer vent routing.

I'm very excited about the Suckerpunch15 coming together so well. The design phase has been a long time running, but the end result is a flat out killer amp. More to come on this amplifier as the cab is completed and the chassis and speaker are installed.

In the meantime, I've started building the prototype of another amplifier I've designed which I'm calling the Toll-Free Express. Preliminary circuit design is complete and the first order of business is fabricating the turret board which will serve as the main "guts" of this amp. Below you'll see the blank G10 turret board material with the board layout template taped down to it as a drilling guide.

And after marking the drilling points, the turret holes were drilled and deburred.

Next comes the task of installing the turrets in each hole and then soldering in the board components.

I'm quite excited about this amp, it should be quite a fun little 10-watt class A head. More to come on this guy as well...

Tube Amp Basics 1 -- Preamp Tube Familiarization

This post is intended as a very basic introduction to preamp tubes. For the sake of simplicity and minimizing confusion, the material here is very top-level. In other words, I will not be getting into in-depth technical discussion here. The purpose is to familiarize the complete noob to the fundamentals of what a preamp tube is and what it does using a 12AX7 as the example.

I'll also give a brief overview of a common gain stage configuration seen in guitar amplifiers and go over identification of schematic symbols in that regard. Again, I'll just be scratching the surface here. A lot of material on this topic starts out by going over very complex analysis. I'll try to keep this as simple as possible by going over the basics in plain English. Obviously, I won't be going over pentode preamp tubes or other odd varieties. Those are less common and the scope of this post is not intended to go over them.

Above is a photo of the most popular variety of preamp tube used in guitar amplifiers: the 12AX7. The 12AX7 is what is known as a noval variety (9-pin) dual triode. What does dual triode mean? In plain English, it means that this tube actually has two distinct, independent amplifier stages inside of its glass envelope.

To further understand this, let's take a look at the elements inside this tube. Pictured below is a common schematic symbol representation of a single triode out of a 12AX7 (one half of the tube).

In the image above, "12AX7" of course refers to the type of tube. "V1a" is the part number as it relates to the schematic. In this example, the part number represents the "a" side (or triode) of valve #1.

Within the schematic symbol, you see representations of three separate elements within the tube. These three elements comprise what we call a triode, hence the name. Starting at the top, this element is known as the plate (or anode in some references). The middle element is known as the grid. And the bottom element is known as the cathode.

Using these three elements, we can create a common preamp gain stage. The gain stage's function is to amplify an AC voltage (like your guitar signal). In many references, this is referred to as a "triode gain stage."

Dual triode tubes like the 12AX7 have two of these triodes within them, hence the name "dual triode." So in practical applications, you can use a single tube to amplify your guitar signal two separate times. Remember how the schematic symbol above only represented one half of the 12AX7 tube? We would call the other half "V1b" to represent the other half, or "b" side of valve #1. It would look identical to V1a. There are of course many different things you can do with these tubes other than gain staging, but the most common use is configuring them as standard gain stages so that is what will be discussed here.

There is one thing about the schematic symbol above that I haven't touched on, and that is the numbering. What do the numbers 1, 2, and 3 represent on the symbol? They represent the corresponding pin numbers on the tube. So pin 1 is V1a's plate (or anode), Pin 2 is V1a's grid, and pin 3 is V1a's cathode.

Let's take a look at the pin configuration of our 12AX7 below:

How do we know which pin is which? Looking at the orientation above, we see that all the pins are evenly spaced from one another except for two of the pins at the bottom. The two bottom-most pins have a larger gap between them. The left of those two pins is pin #1 and the numbering moves sequentially as you go clockwise around the tube. So the bottom-most pin on the right is pin #9.

So wait a minute! If this is a dual triode, meaning the tube contains a total of 6 elements, then how come we have NINE pins in total here??

The other elements I have not addressed yet are the heater filaments. The heater filaments are the ones you see light up nice and pretty when your tubes are powered up. Their job is to heat up the cathode, which I'll get into in a bit. But inside the tube, each triode has it's own heater filament. Triode "a" (represented by pins 1, 2, and 3) claims ownership of the heater connected to pin 4. Triode "b" has the heater connected to pin 5. Moving along, pin 6 is the plate (or anode) of triode "b," pin 7 is the grid, and pin 8 is the cathode. So pins 6, 7, and 8 would comprise "V1b" using our schematic example above.

The final pin, pin #9 is essentially a center tap connection to the two heater filaments inside the tube.

Why does the heater filament heat the cathode?

If you don't care about technical stuff just yet, you can skip this brief explanation. However, if you're curious, keep reading.

Inside the tube, we're dealing with the manipulation of electrons inside a vacuum. The cathode element of our triode has a coating covering it. When the heater filament heats up the cathode, electrons begin to boil off of this coating. There is a specific purpose for doing this, but I will not go into detail here. Just know that this is where your electrons are coming from in a triode amplifier.

Summary

So we now know that a 12AX7 is a nine pin tube containing two independent amplifier stages known as "triodes." Each triode is comprised of three separate elements, the plate (or anode), the grid, and the cathode. The other elements within the tube are the heater filaments whose sole purpose is to make the cathodes really hot so that electrons can boil off of them.

Let's take a look at how a standard gain stage is configured within a guitar amplifier. In the picture below, you'll see our V1a triode that we looked at before, but now it's connected to several other components.

For now, we won't get into what everything is doing in this configuration. Let's just identify what we are looking at here.

As you can see, the guitar signal is entering the triode from the left and going into the grid element. R1 is a resistor and is commonly referred to as a "grid leak" resistor. R2 is commonly referred to as a "grid stopper" resistor. R3 is commonly referred to as the "plate resistor" or "anode resistor." R4 is commonly referred to as the "cathode resistor."

C1 is a capacitor that is commonly known as the "coupling capacitor." And C2 is commonly known as the "cathode bypass capacitor."

All of these components contribute to the operation of the gain stage as well as the tone shaping that occurs within the stage. Sometimes you will see additional components in a stage. For simplicity's sake, I've kept the representation basic.

As you can see from the image, the amplified guitar signal exits the tube from the plate and travels through the coupling capacitor C1. From there it can go into another gain stage (like V1b), a tone stack, a gain knob, or anywhere else the amp designer has in mind at this particular point in the amplifier.

In a future post, I will go into more detail about what is happening in a gain stage like this as well as how the guitar signal's gain and tone is shaped and affected by the values of these components. For now, I hope you've gotten a good basic understanding of preamp tubes from this post.